Changed the 'mean' labels to 'upper'

These values, previously named p90, p95, p99, are actually equivalent to
Statsd's upper_* metrics--not mean_*.

I'm not actually sure how to implement calculating the mean up to the
upper_* value in this program--the algorithm used by statsite doesn't
store an array of sorted metrics in its quantile calculations. (which is
part of the reason it's so fast...)